• To name two contrasting theories of perception

• To explain what is meant by the phrase ‘Top Down’ processing

• To Outline Richard Gregory’s theory of perception.

Bottom Up / Direct theory

Top Down/ indirect theory

• Top Down Processing

• Our brain interacts with ‘lower’ functions

• To name two contrasting theories of perception

• To explain what is meant by the phrase ‘Top Down’ processing

• To Outline Richard Gregory’s theory of perception.

“Perception is not determined simply by stimulus patterns. Rather it is a dynamic

searching for the best interpretation of the available data...which involves going beyond

the immediately given evidence of the senses”

Gregory (1966)

What does perception mean to you?

Definition:A cognitive process that allows sensory information (sights, sounds, touch, taste, smell) to be interpreted and understood.

• Gregory proposed that sensory information is incomplete or ambiguous.

• We use our prior “experience of the world to shape how we perceive” stimuli we encounter in it.

• We use activated conceptual schemas and memory networks (stored knowledge) to shape perceptions or interpret sensory input.

• This can sometimes lead to incorrect perceptions.

• He used visual illusion research to support many of his theoretical propositions.

Gregory’s Top-Down Theory

Expectations

Perceptual set occurs through expectations, where people perceive what they expect to based on previous experiences.

Expectations

Bruner and Mintern (1955) showed an ambiguous figure to participants that could be seen as either the letter ‘B’ or the number ’13’.

Participants who saw the figure surrounded by the consecutive numbers 12 and 14 perceived it as 13, while those who saw it flanked by the letters A and C saw it as the letter B

This suggests that the context in which stimuli are viewed creates an expectation that shapes actual perception.

The Role of Expectations in Perception

• The hollow face illusion

• ‘red’ wine ( E-numbers, an edible adventure, 9 minutes in)

• http://www.youtube.com/watch?v=r_fg6Aq-w0c

Expectations

Brochet (2002) gave 54 experienced wine tasters an array of white wines, which they described as ‘fresh’ and so on.

He then gave them an array of red wines to evaluate, some of which were white wines, made to look red by the addition of a tasteless, odourless additive.

Not one expert spotted the frauds, describing them in terms reserved for red wines (such as ‘intense’ ‘spicy’).

Because they had increased knowledge of wine, they were more influenced by colour. The colour created an expectation, which determined actual perception.

Why do you think Brochet had his participants rate genuine white wine first?

Expectations

Leeper (1935) showed participants an ambiguous picture, which could either be seen as a young or an old woman.

Participants previously given a description or picture of a young woman perceived a young woman, while those described or shown an old woman saw an old woman – showing the influence of expectations.

Expectations

See Palmer’s research on p75. and the role of hypothesis testing

Gregory’s Top Down/ Indirect Theory

• The information coming in from your eyes isn’t good enough to form an accurate perception of the world around us.

• We use knowledge we already have to make a ‘best guess’ about what we are sensing.

It is a constructivist theory

• We construct our perceptions by combining the information received by our eyes (direct data) with sorted knowledge in the brain.

Gregory’s Indirect theory says perception is based on three things

1. Sensory Data2. Knowledge stored in the brain3. An inference or hypothesis about what is out

there

Evidence to support Gregory’s theory

• Visual illusions offer the main support for Gregory’s theory- particularly the MULLER- LYER illusion

Which of the two vertical line segments is longer? Although your visual system tells you that the left one is longer, a ruler would confirm that they are equal in length.

What about the Muller-Lyer illusion?

Explaining the Muller- Lyer

Our experience of the world tells us that the edge of the building is closer than the rest of the building

Our experience of the world tells us that the corner of the room is further away from us than the rest of the room.

Because we ‘know’ the distance is different but they are the same size on our retina, they must actually be different sizes!

Size and distance

• The size something REALLY is depends on 3 things

– The size of the image on your retina (how big it looks)

– How far away it is– Roughly how big they usually are

How does Gregory explain visual illusions?

• Visual illusions are the result of misapplied hypotheses

• When you ‘see’ something you formulate a hypothesis when formulating your perception of that object.

• These normally work in the ‘real world’ but illusions are designed to be tricksy!

The Ponzo illusion

If it is based on expectations…..

• Then people who grew up without corners shouldn’t fall for it!

• Segall et al (1963) found that people who did not live in a carpentered world did not fall for it (as much)

Segall (1963) found that Africans living in open country, where occasional vertical objects were important features, were susceptible to the horizontal-vertical illusion, while those living in dense jungle, were less likely to see the illusion

Evaluation of GregoryExplains how we deal with ambiguous situations

There is evidence to support it

X Largely based on artificial settings and the odd occasion where perception fails

X We continue to ‘see’ visual illusions even when we ‘know’ how they work- why don’t we alter our existing knowledge or formulate new hypotheses.

• To name two contrasting theories of perception

• To explain what is meant by the phrase ‘Top Down’ processing

• To Outline Richard Gregory’s theory of perception.